1. Field of the Invention
The present invention relates generally to holographic memory systems, and more particularly, to sensing angular orientation of a holographic medium in a holographic memory system.
2. Related Art
Developers of information storage devices continue to seek increased storage capacity. As part of this development, memory systems employing holographic optical techniques, referred to herein as holographic memory systems, have been suggested as alternatives to conventional memory devices.
Typically, holographic memory systems read/write data to/from a photosensitive storage medium. Such systems typically access holographic representations (i.e., holograms) substantially throughout the spatial extent of the storage medium. This allows holographic memory systems to advantageously store a large amount of data.
Holographic memory systems may be designed to record data as single bits of information (i.e., bit-wise data storage). See McLeod et al. “Micro-Holographic Multi-Layer Optical Disk Data Storage,” International Symposium on Optical Memory and Optical Data Storage (July 2005). Holographic memory systems may also be designed to record an array of data that may be a 1-dimensional linear array (i.e., a 1×N array, where N is the number linear data bits), or a 2-dimension array commonly referred to as a “page-wise” memory system. Page-wise memory systems may involve the storage and readout of an entire two-dimensional representation, (i.e., a page) of data.
Holographic memory systems typically involve the three-dimensional storage of holograms as a pattern of varying refractive index and/or absorption imprinted into the storage medium. In general, holographic memory systems operate to perform a data write (also referred to as a data record or data store operation, simply “write” operation herein) by combining two coherent light beams at a particular point within the storage medium. Specifically, a data-encoded light beam is combined with a reference light beam to create an interference pattern in the photosensitive storage medium. The interference pattern induces material alterations in the storage medium to form a hologram. The formation of the hologram is a function of the relative amplitudes, phase, coherence and polarization states of the data-encoded and reference light beams. It is also dependent on the relative wavelength of the incident beams as well as the three dimensional geometry at which the data-encoded and reference light beams are projected into the storage medium.
Holographically-stored data is retrieved from the holographic memory system by performing a read of the stored data (also referred to as a data reconstruction operation, simply “read” operation herein). The read operation is performed by projecting a reconstruction or probe beam into the storage medium at the same angle, wavelength, phase and position as the reference beam used to record the data, or compensated equivalents thereof. The hologram and the reconstruction beam interact to reconstruct the data beam. The reconstructed data beam is then detected by a sensor, such as a photo-detector, sensor array, camera, etc. The reconstructed data is then processed for delivery to an output device.
As noted, illumination of a holographic medium with coherent light beams having precise incidence angles, wavelengths, phases, positions, etc., allows for accurate operation of the holographic memory system. Accordingly, it is desirable to ensure that the holographic memory system is properly aligned and free of error-causing defects to achieve correct illumination of the holographic medium.